Not GP but one time we did a cyanation reaction (attaches a -CN group) in the pilot plant involving trimethylsilyl cyanide (TMS-CN). We had what looked honest to god like an 8lb propane tank of the stuff ("cyanide and cyanide accessories" lol), with an air-free delivery siphon, typical for air-free, pyrophoric, and toxic reagents.

Normally we'd do this in the high potency suite but the reaction scale was too big so we did it in the conventional GMP pilot plant. Bunny suits and cyanide meters for all three chemists.

I calculated that dose-wise, it was enough to kill tens of thousands, were it directly delivered, however if it were merely vaporized, it would only kill those in the immediate area, and would hydrolyze quickly. There's a great article linked elsewhere in thread about the ineffectiveness of chemical weapons, I've added it for convenience because it's a great read. Air is big, the distance-cubed law really dilutes any toxic agent beyond dangerous limits quite fast (comparing pound for pound lethality of conventional explosives).

https://acoup.blog/2020/03/20/collections-why-dont-we-use-ch...

Maybe azides.